Differentiated cell population of endothelial cells derived from human pluripotent stem cells, composition, system, kit and uses thereof

ABSTRACT

The present disclosure relates to a differentiated cell population of endothelial cells derived from human pluripotent stem cells. The present invention also relates to a composition, a system and a kit comprising those cells and uses thereof. The present disclosure also described the combination of arterial ECs derived from human pluripotent stem cells with a microfluidic system to create a vascular kit for high-throughput drug screening and/or toxicology analysis. This technology may find particular use for the identification of drugs that may have a fetal cytotoxic effect.

TECHNICAL FILED

The present disclosure relates to a differentiated cell population of endothelial cells derived from human pluripotent stem cells. The present invention also relates to a composition, a system and a kit comprising those cells and uses thereof.

BACKGROUND

The development of new technologies and tools for the rapid toxicological profiling of chemical/pharmaceutical substances, at cellular levels, are of great need to reduce and refine the use of animals in research. Vascular cells control the permeability of blood vessels, inflammation and immunity, cell growth, among other key functions, which have an important impact in the homeostasis of the human being.

Vascular cells derived from human pluripotent stem cells (hPSCs) represent a potential cell source for vascular kits^(1, 2). The use of “embryonic” ECs may represent an opportunity to screen toxicity of compounds that affect embryonic vasculature. ECs have been derived from human embryonic stem cells (hESCs) using several methodologies such as embryoid bodies (EBs) which recapitulates in vivo embryogenesis^(3, 4), a mixture of EBs with 2D or 3D culture systems^(1, 5-7) and co-culture with cell lines⁸⁻¹⁰.

However, there is no report showing the specification of hESC-derived ECs into arterial, venous or lymphatic sub-phenotypes either in vitro or after transplantation in animal models. This is important for the development of vascular kits to assess vascular toxicity and to target specific vascular vessels for therapeutic use. During embryonic development, specification into arterial-, venous- or lymphatic-derived ECs is defined at gene level and is mediated by several signaling pathways including VEGF, Notch and ephrin before circulation begins^(11, 12). Studies in mouse have shown that ephrin B2 and its receptor ephB4 are differentially expressed in arterial and venous ECs, respectively, before the onset of circulation in the developing embryo¹³. After the onset of the circulation, the distinct hemodynamic forces found in arteries and veins, such as blood flow rate, direction and pressure, can be a major driver in the specification and maturation of the ECs^(11, 12). Indeed, hemodynamic forces as shear stress have the capacity to program or redirect the specification of blood vessel type during development^(11,12).

The development of vascular kits requires the development of microfluidic platforms to screen multiple compounds in a high-throughput while the cells are exposed to shear stress forces typically found in vivo. Only recently, researchers have replicated the circular cross-section of blood vessels in microfluidic devices¹⁴⁻¹⁶. However, so far, these tools have not been used in the context of drug screening/toxicology assessment.

SUMMARY

The present disclosure relates to a differentiated cell population of endothelial cells derived from human pluripotent stem cells wherein a portion of the said endothelial cells express ephrin B2. These differentiated cell population is particular useful in the screening embryonic vascular toxicity or therapeutic compounds.

The pluripotent stem cells used in the present disclosure are obtained without having to recur to a method necessarily involving the destruction of human embryos, namely with the use of iPS cells.

Better results could be achieved wheb the portion of differentiated cell population of endothelial cells derived from human pluripotent stem cells has at least 20% of ephrin B2, preferably at least 25%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 85%, 90%, 95%, 100%, more preferably 50% to 75% of the cells express ephrin B2. These cells may have the ability to form cord-like structures when cultured in the basement membrane Martrigel.

In order to optimize the useful in the screening embryonic vascular toxicity or therapeutic compounds, the cell population of endothelial cells may further express the following marker Ac-LDL.

In order to optimize the useful in the screening embryonic vascular toxicity or therapeutic compounds, the cell population of endothelial cells may further express one of the following markers: vWF, CD31, CD34, vascular endothelial cadherin, Flk-1/KDR.

In order to optimize the useful in the screening embryonic vascular toxicity or therapeutic compounds, the cell population of endothelial cells may has a high expression of one of the following arterial endothelial cell genes: jagged 1—JAG1, jagged 2—JAG2, ephrin B1, Hey-2.

In order to optimize the useful in the screening embryonic vascular toxicity or therapeutic compounds, the cell population of endothelial cells may has a high expression of one of the following arterial endothelial cell genes: receptor protein tyrosine phosphatase, T-cell acute lymphocyte leukemia, N-cadherin, angiopoietin 1, DNA-binding protein inhibitor ID-1.

In order to optimize the useful in the screening embryonic vascular toxicity or therapeutic compounds, the cell population of endothelial cells may has a low expression of venous genes such as EphB4, lefty-A and lefty-B.

Other aspect of the present disclosure is related with a differentiated cell population of endothelial cells derived from human pluripotent stem cells wherein a portion of the said endothelial cells express: receptor protein tyrosine phosphatase, T-cell acute lymphocyte leukemia, N-cadherin, angiopoietin 1, DNA-binding protein inhibitor ID-1. These differentiated cell population is particular useful in the screening embryonic vascular toxicity or therapeutic compounds.

Another aspect of the present disclosure is differentiated cell population of endothelial cells derived from human pluripotent stem cells wherein a portion of the said endothelial cells express at least one of the following markers: vWF, CD31, CD34, vascular endothelial cadherin (VE-CAD), Flk-1/KDR for the use in the screening embryonic vascular toxicity or therapeutic compounds.

Better results could be achieved wheb the portion of differentiated cell population of endothelial cells derived from human pluripotent stem cells has at least 20% of the said cells express at least one of the following markers: vWF, CD31, CD34, vascular endothelial cadherin (VE-CAD), Flk-1/KDR, preferably at least 25%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 85%, 90%, 95%, 100%. These cells may have the ability to form cord-like structures when cultured in the basement membrane Martrigel.

Other aspect is the use in medicine of the differentiated cell population of endothelial cells derived from human pluripotent stem cells. Namely, using the cell population in the screening embryonic vascular toxicity or therapeutic compounds.

Other aspect is a composition comprising the cell population above described, namely a pharmaceutical composition.

Other aspect is a kit for use in screening vascular toxicity or therapeutic compounds comprising the cell population above described.

Other aspect is a fluidic system for use in screening therapeutic drugs or embryonic vascular toxicity, comprising a channel with a millimeter or micrometer dimension and a differentiated cell population of endothelial cells as described in the present disclosure.

In an embodiment of the system, the cells cultured under physiologic shear stress are cells seeded and exposed to the media flow, and may produce glycocalyx. In another embodiment the media flow of the system may be above 1 dyne/cm2, preferably is above 4 dyne/cm2, preferably 20 dyne/cm2.

In an embodiment of the system, the channel may comprises poly(dimethylsiloxane).

In other preferred embodiment of the system channel may has:

-   -   a length of 0.1-1.0 cm, preferably 0.5 cm;     -   and an inner diameter higher than 200 μm, more preferably         200-900 μm, more preferably 400-600 μm.

In another embodiment the system may further comprises plasma such as argon. Preferably, the system may further comprises gelatin, collagen, or fibronectin, or fibrin, or matrigel or mixtures thereof.

In another embodiment the media flow of the system is above 1 dyne/cm2, preferably is above 4 dyne/cm2, preferably 20 dyne/cm2.

In another embodiment, the cells of the system above described may be the cell population above described.

Other aspect is a device for use in screening vascular toxicity or therapeutic compounds comprising: endothelial cells derived from human pluripotent stem cells as above descrided; and a fluidic system as above described. The device could be use in screening embryonic vascular toxicity or therapeutic compounds, or in screening embryonic arterial endothelial cell toxicity or therapeutic compounds or in screening embryonic arterial endothelial cell toxicity or therapeutic compounds in conditions that mimic the in vivo conditions, or use in the screening of antitumor or anticancer drugs.

In an embodiment, the compounds of the device may be selected from the following group: danazol, chlorpromazine hydrochloride, ellipticine, 3′,4′-dichlorobenzamil, fluphenazine dihydrochloride, 7-cyclopentyl-5-(4-phenoxyl)phenyl-7H-pyrrolo[2,3-d]pyrimidin-4-ylamine, among others.

Other aspect is a method of promoting the viability of arterial endothelial cells comprising differentiation from human pluripotent stem cells with the following steps:

-   -   culturing embryoid bodies in suspension for 4 days in         differentiation medium comprising VEGF165;     -   seeding the said embryoid bodies in a cell culture and         sequentially exposing the cells to VEGF plus thymosin 134 for 3         days and VEGF plus thymosin 13 for 4 days and SB431542 for 11         days;     -   isolating CD31+ cells and culturing these cells in endothelial         cell medium containing SB431542.

The reported methodology applies where the pluripotent stem cells are induced pluripotent cells that do not require the death of the embryo.

In an embodiment, of the method the human pluripotent stem cells were differentiated in conditions that Shh and Notch signaling is activated.

In other embodiment, of the method the human pluripotent stem cells were differentiated in the presence of DLL4 and purmorphamine preferably, 100 ng/mL of DLL4, and 1 μM of purmorphamine.

In other embodiment, of the method the predetermined conditions comprise a seeding density of 15,000 cells per cm2 in a gelatin-coated dish.

In other embodiment, of the method further comprising adding to the cell population of CD31+, SB431542 with a concentration higher than 1 μM, preferably 5 μM-10 μM.

In other embodiment, of the method further comprising adding to the cell population of CD31+ miRNAs by a transfection agent. Preferably the said agent may be a nanoparticle.

In other embodiment, of the method the said endothelial cell medium for culturing CD31+ cells has at least one of the following growth factors: VEGF, PDGF, angiopoietin (Ang), ephrin (Eph), fibroblast growth factor (FGF), placental growth factor (PIGF), transforming growth factor β-1 [(TGF)-β-1], cytokines, erythropoietin, thrombopoietin, transferring, insulin, stem cell factor (SCF), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony stimulating factor (GM-CSF) and their mixtures.

In other embodiment, of the method further comprising co-culturing CD31+ cells with arterial endothelial cells.

All the embodiments described are obviously combinable.

The present disclosure also described the combination of arterial ECs derived from human pluripotent stem cells with a microfluidic system to create a vascular kit for high-throughput drug screening and/or toxicology analysis. This technology may find particular use for the identification of drugs that may have a fetal cytotoxic effect.

DESCRIPTION OF THE DRAWINGS

The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of invention.

FIG. 1—Specification of hPSC-derived ECs into arterial phenotype. (A) EC marker expression and functionality of hESC-derived ECs. In all figures bar corresponds to 50 μm. (B) Microarray analysis showing that hESC-derived ECs share many arterial genes as shown in hUAECs and hAECs. The list of genes correlates with the heat map. (C) qRT-PCR analysis for arterial and venous markers in hESC-derived ECs at passage 4. The expression of the same genes for hUAECs and hAECs is displayed. Results are mean±SEM (n=4). (D) Expression of venous and arterial genes in CD31⁺ cells differentiated in basal media or media containing both Shh and Notch activators (purmorphamine, 1 μM and DLL4, 100 ng/mL). Results are mean±SEM (n=4). (E) Expression of Notch and Shh genes in CD31⁺ cells after isolation. Results are mean±SEM (n=4). (F) Expression of arterial marker EphB2 in CD31+ cells after treatment with chemical activators and inhibitors of Shh and Notch signaling pathways. Shh inhibitor is cyclopamine (5 μM) and Shh activator is purmorphamine (1 μM). Notch inhibitor is L685458 (1 μM) and Notch activator is DLL4 (100 ng/mL). (G) qRT-PCR analysis for embryonic endothelial markers in hESC-derived ECs at passage 4, hUAECs, mouse embryonic ECs at day 12.5 (mAEC E12.5) and postnatal day 1 (p1). (H) Microarray analysis showing that hESC-derived ECs express embryonic genes not present in fetal hUAECs or adult hAECs. The list of genes correlates with the heat map. (I) Variation of intracellular Ca²⁺ in FURA-2-loaded cultured hESC-derived ECs, HUAECs or HUVECs in response to several agonists. Traces are representative of 6 independent experiments for each condition. In C, D, E and F, gene expression was normalized by the expression of GAPDH. In all figures, *P<0.05, **P<0.01, ***P<0.001.

FIG. 2—Effect of fluidic shear stress on hPSC-derived ECs. (A) Cell alignment and expression of endothelial (VECad) and arterial (EphB2) markers in hESC-derived ECs. Bar corresponds to 50 μm. (B) Flow cytometry analysis of hESC-derived ECs cultured for 7 days in static and flow (20 dyne/cm2) conditions. Percentages of positive cells were calculated based in the isotype controls (grey plot) and are shown in each histogram plot. EphB2 expression on HUAECs cultured under static or flow conditions are also shown for reference. (C) qRT-PCR analysis for venous and arterial markers in hESC-derived ECs (data was normalized by the housekeeping gene GAPDH) cultured for 7 days in static or flow (4 or 20 dyne/cm2) conditions. Results are mean±SEM (n=4). (D) Expression of HSPG in hESC-derived ECs and HUAECs in static and flow conditions for 7 days at 20 dyne/cm2. Bar corresponds to 50 μm.

FIG. 3—hPSC-derived ECs as a model to evaluate vascular toxicity. (A) Expression and localization of VECad in hESC-derived cells and HUAECs cultured in medium supplemented with terbinafine. Cells were cultured under flow (20 dyne/cm2), in medium without terbinafine, for 7 days, after which the cells were cultured in medium supplemented with terbinafine (0.1 or 1 μM) for 24 h, under flow. Bar corresponds to 50 μm. (B) Expression of genes involved in inflammation (ICAM-1; E-selectin), oxidative stress (HO-1), vascular modulation (eNOS) and vascular injury (DDAH1 and DDAH2; genes that encode for enzymes that degrade ADMA) in hESC-derived ECs and HUAECs cultured in medium supplemented with terbinafine. Results are mean±SEM (n=4). *P<0.05, **P<0.01, ***P<0.001. (C) Quantification of ADMA and vWFpp:vWF by ELISA in hESC-derived ECs and HUAECs.

FIG. 4—High-throughput screening (HTS) to identify compounds with embryonic vascular toxicity. (A) Schematic representation of the HTS assay. (B) Small molecules identified after the analysis of the primary screen. The hits have higher cytotoxicity against hESC-derived ECs than HUAECs. (C) Dose-response curve for HUAECs and hESC-derived ECs exposed to 7-Cyclo. Results are mean±SEM (n=4).

FIG. 5—Validation of the hit 7-Cyclo in static and flow conditions. (A) Macroscopic view of the PDMS microfluidic system (the microchannels have a diameter of 900 μm and an average length of 0.5 cm) and fluorescent images of microchannel cross-sections showing that ECs can populate the inner surface of the microfluidic channel after 48 h and be stable for at least 7 days at 20 dyne/cm2. Scale bars are 50 μm. (B) Cell organization and morphology of hESC-derived ECs and HUAECs before and after incubation with 7-Cylo for 24 h in a microfluidic system. Bar corresponds to 50 μm. (C) Expression of genes involved in inflammation (ICAM-1; E-selectin), oxidative stress sensing (HO-1), vascular modulation (eNOS) and vascular injury sensing (DDAH1 and DDAH2) in hESC-derived ECs and HUAECs after 24 h incubation with 0 or 1 μM of 7-Cyclo. Results are mean±SEM (n=4). *P<0.05, ***P<0.001, ****P<0.0001. (D) Quantification of ADMA and vWFpp:vWF by ELISA in hESC-derived ECs and HUAECs after 24 h incubation with 1 μM of 7-Cylo. Results are mean±SEM (n=6). *P<0.05, **P<0.01, ****P<0.0001.

FIG. 6—Mechanism of 7-Cyclo. (A) Expression of genes involved in inflammation (ICAM-1; E-selectin), oxidative stress sensing (HO-1), vascular modulation (eNOS) and vascular injury sensing (DDAH2) in mouse aortic endothelial cells (mAEC) at E12.5 and p.1 (post-natal day1) after 24 h incubation with 0 or 1 μM of 7-Cyclo. Results are mean±SEM (n=4). **P<0.01, ***P<0.001. (B) Microarray analysis showing the expression of tyrosine kinases in hESC-derived ECs, hUAECs and hAECs. Some of the tyrosine kinases are higher expressed in hESC-derived ECs than in hUAECs or hAECs (zoom of the microarray). (C) Validation of the expression of tyrosine kinases by qRT-PCR. Gene expression was normalized by the expression of GAPDH. Results are mean±SEM (n=4). *P<0.05. (D) Kinase activity on hESC-derived ECs and HUAECs after incubation with variable doses of 7-Cyclo. Luminescence is inversely related to kinase activity. Results are mean±SEM (n=6). **P<0.01, ***P<0.001.

FIG. 7—Induction of endothelial differentiation on hPSCs. (A) Scheme showing the differentiation protocol. (B) Expression of CD31 marker, as quantified by FACS, in hESCs differentiated for 18 days in media supplemented with VEGF165, Tβ4 and SB431542 at different times, or only media without supplements. Values indicate mean±SEM from 3 independent experiments. (C) Gene expression on hESCs during the induction of endothelial differentiation. Gene expression was evaluated by qRT-PCR and the values normalized by the corresponding gene expression observed in HUVECs, with the exception of Oct-4, which was normalized by the corresponding gene expression in undifferentiated hESCs. Results are mean±SEM (n=4). (D) Gene expression on hESC-derived ECs. hESC-derived ECs were obtained from CD31+ cells isolated by MACS and differentiated for 3 passages (approximately 22 days after cell seeding). Gene expression was normalized as in C. (E) Flow cytometry analysis of hESCderived ECs. Percentages of positive cells were calculated based in the isotype controls (grey plot) and are shown in each histogram plot.

FIG. 8—Induction of vascular differentiation on hESCs. (A) Scheme summarizing the differentiation protocols (Prot1-Prot7). (B) Expression of CD31 marker in hESCs differentiated for 18 days. Values indicate mean±SEM from 3 independent experiments. (C) Gene expression in hESC-derived EC cells differentiated by some protocols, as evaluated by qRT-PCR. Gene expression in each experimental group was normalized by the corresponding gene expression observed in HUVECs. Results are mean±SEM (n=3-4).

FIG. 9—Characterization of hESC-derived ECs, HUVECs and HUAECs. (A) Time-course proliferation of hESC-derived ECs. (B) hESC-derived ECs (Prot7, passage 3, approximately 22 days after CD31+ cell seeding) have cobblestone morphology and do not express α-SMA, a smooth muscle cell marker. (C) Expression of arterial (EphB2) and venous (Lefty) markers in HUVECs and HUAECs. Bar corresponds to 50 μm.

FIG. 10—Characterization of hIPS-derived ECs (hIPSC—Human Induced Pluripotent Stem Cell). CD31+ cells isolated by MACS were plated and differentiated for 4 passages (approximately 17 days after cell seeding) using the differentiation protocol previously used for hESCs. (A) Flow cytometry analysis of hIPS-derived ECs. Percentages of positive cells were calculated based in the isotype controls (grey plot) and are shown in each histogram plot. (B) EC marker expression and functionality of hESC-derived ECs. In all figures bar corresponds to 50 μm. (C) qRT-PCR analysis for EC markers. Vascular gene expression in each experimental group was normalized by the corresponding gene expression observed in HUVECs, with the exception of Oct-4, which was normalized by the corresponding gene expression in undifferentiated hIPS. In the graphs corresponding to vein and arterial gene expression the genes were normalized by the expression of GAPDH. Results are mean±SEM (n=4). *P<0.05, **P<0.01, ***P<0.001.

FIG. 11—Expression of arterial and venous genes during the inductive and spontaneous differentiation protocol. (A) Scheme summarizing the inductive and spontaneous differentiation protocols. (B-C) Expression of arterial and venous markers in hESCs differentiated for 18 days using the inductive (B) or spontaneous (C) differentiation protocols. Values indicate mean±SEM, n=4. (D) Effect of activation and inhibition of Shh and Notch signaling pathways in the expression of EphB2 in CD31+ cells. Shh inhibitor is cyclopamine (5 μM) and Shh activator is purmorphamine (1 μM). Notch inhibitor is L685458 (1 μM) and Notch activator is DLL4 (100 ng/mL).

FIG. 12—Expression of arterial and venous EC markers in HUAECs cultured on static and flow conditions (7 days), as evaluated by immunocytochemistry (A) and qRT-PCR analysis (B). In A, bar corresponds to 50 μm. In B, results are Mean±SEM (n=3-4).

FIG. 13—Effect of physiological shear stress in the subphenotype of hESC-derived ECs. (A) Cells cultured under static or flow conditions for 7 days do not express the venous EC marker Lefty. Bar corresponds to 50 μm. (B) hESC-derived ECs and HUAECs cultured under static conditions do not express heparan sulfate proteoglycan (HSPG), while both cells express HSPG under flow conditions. Bar corresponds to 50 μm.

FIG. 14—Hit validation. After being selected from the primary screen, the identified compounds (hits) were subjected to a half-logaritimic dilutions in order confirm the positive hits and to assess the optimal concentration inducing cytotoxicity.

FIG. 15—Quantification of sprouts and length of cord-like structures in hESC-derived ECs (A), HUAECs (B) and HUVECs (C) cultivated on top of Matrigel after 0, 3 and 20 h, with or without 7-Cyclo. Results are mean±SEM (n=8). *P<0.01, ****P<0.0001. Bar corresponds to 50 μm.

DETAILED DESCRIPTION

The present disclosure relates to differentiated cell population of endothelial cells derived from human pluripotent stem cells.

The said pluripotent stem cells used in the present invention are obtained without having to recur to a method necessarily involving the destruction of human embryos.

In the present disclosure is presented a platform for the high-throughput screening of compounds that might interfere with embryonic vascular development. Initially, conditions were screened for the differentiation of hPSCs (human plurpotent stem cells) into embryonic arterial ECs followed by their maturation under flow shear stress. In static conditions, arterial ECs express arterial genes such as JAG1, ephrin B1 and Hey-2 and the arterial ephrin receptor B2 (EphB2).

In flow conditions, the cells align in the direction of the flow and further up-regulate the expression of arterial genes. The process is likely mediated by heparan sulfate proteoglycan (HSPG), a component of glycocalyx, which is activated by fluidic shear stress. The utility of embryonic arterial ECs cultured under flow conditions for toxicological assessment was then demonstrated. The higher sensitivity to cytotoxic compounds such as terbinafine of hESC-derived ECs cultured under physiologic shear stress than cells cultured in static conditions was shown. Additionally, using a high-throughput assay in combination with in vivo data, was identified the 7-cyclopentyl-5-(4-phenoxyl)phenyl-7H-pyrrolo[2,3-d]pyrimidin-4-ylamine (7-Cyclo) as a cytotoxic compound of embryonic ECs.

The disclosed platform is a powerful platform for drug screening and to study embryonic vascular biology under physiologic conditions.

In the disclosed subject matter is described the combination of arterial ECs derived from human pluripotent stem cells with a microfluidic system to create a vascular kit for high-throughput drug screening and toxicology analysis. This technology may find particular use for the identification of drugs that may have a fetal cytotoxic effect. Furthermore, the principles defined in this work may be applied to iPSCs in order to create personalized kits for drug screening.

hPSCs can differentiate into arterial ECs. The ECs were characterized at protein level by the expression of EphB2 and the absence of venous Lefty 1/2 and lymphatic podoplanin markers. At gene level, the cells express most of arterial markers shown by HUAECs and HAECs. The arterial specification is mediated in part by Shh and Notch signaling pathways. Results show that the activation of both pathways is required to enhance arterial specification, as previously shown^(47,48,12.) The activation of both Shh and Notch signaling pathways from the very beginning of the differentiation procedure (from day 0 up to day 18; before the isolation of CD31⁺ cells) increased significantly the percentage of cells (from 18% to 36%) already committed into an arterial sub-phenotype. Furthermore, the specification of hPSCs into arterial ECs largely occurs after the isolation and differentiation of CD31⁺ cells. The arterial ECs differentiated in this study have an “embryonic” phenotype. Although, a defined set of embryonic EC markers have not been identified so far, in the present disclosure was also identified receptor protein tyrosine phosphatase μ (PTPRu)²⁴, T-cell acute lymphocyte leukemia 1 (Tall)^(25, 26), and some cadherins^(27, 28), among others, as putative markers of embryonic ECs, based in gene microarray analyses on human ECs as well as gene expression results in mouse embryonic ECs.

The present disclosure also show that fluidic shear stress enhanced the maturation of arterial ECs, and preferably in the maturation of arterial ECs and in the induction of HSPG (shown by the up regulation in the expression of ephrins (1 and 2), notch receptors (1 to 4), and notch ligands (Jagged1 and delta-like ligand 3) and the induction of HSPG. Previous studies have supported the idea that hemodynamic forces have the capacity to program or redirect the specification of blood vessel type during development^(11,12). ECs not only have the ability to sense hemodynamic forces, but they have the ability to discriminate between different types of biomechanical stimuli. In other embodiment of the present disclosure, was shown that hESC-derived ECs cultured in flow conditions (20 dyne/cm2) aligned in the direction of the flow and express significantly higher levels of arterial markers. Although not demonstrated, HSPGs may mediate this maturation effect. Recent data demonstrated that HSPG is a mechanosensor mediating shear stress-induced EC differentiation from mouse embryonic stem cell-derived ECs49. HSPG is a part of the endothelial glycocalyx, which is only expressed in flow conditions and absent in static conditions³³. It is conceivable that HSPGs are physically displaced when exposed to shear and the displacement transmitted to the intracellular machinery. It has been suggested that HSPGs are physically activated (direct or not) to actin and nitric oxide synthase mediating the mechanotransduction process³¹. These intracellular processes may contribute for the maturation of the cells under shear stress.

The results further show that hESC-derived ECs cultured under flow conditions can be used to assess vascular toxicity while showing higher sensitivity to vascular toxic compounds.

For proof of concept was used terbinafine an antifungal drug with anti-angiogenesis and anti-tumoral activity^(34, 50). Terbinafine inhibits endothelial cell migration by inhibiting kinases in the Rho-kinase pathway³⁴. Results show that hESC-derived ECs cultured under flow respond to very low concentrations of terbinafine (0.1 μM). This was correlated with an up-regulation of oxidative-sensing and inflammatory genes, down-regulation of genes (DDAH1 and DDAH2) encoding enzymes that degrade an inhibitor (ADMA) of nitric oxide synthase, an increase in the secretion of ADMA and vWF pro-peptide, markers of EC injury.

In this present disclosure was identified an embryonic arterial EC inhibitor, 7-Cyclo, by high-throughput screening, which was further validated by a dose-response study and cell culture under flow conditions. 7-Cyclo is a Src family tyrosine kinase inhibitor. 7-Cyclo (20 μM) has been reported to interfere with angiogenic sprouting and disrupt blood vessel formation in Xenopus embryos, although it was unclear whether such effect was related to the embryonic stage of the vasculature or if any vasculature could have the same consequences⁵¹. Furthermore, in vitro, 7-Cyclo (1 μM) inhibits HUVECs and lymphatic EC tube formation⁵¹, and it is an inhibitor of lymphangiogenesis⁵². Results indicate that hESC-derived ECs exposed to medium supplemented with 7-Cyclo (1 μM) for 24 h under flow conditions show significant alterations in cell morphology, up-regulation of inflammatory genes, and secretion of vascular injury markers. This effect is higher on hESC-derived ECs than HUAECs. Similar results were obtained for mouse embryonic ECs and post-natal ECs, i.e., mouse embryonic ECs were sensitive to the toxicity of 7-Cyclo while post-natal ECs show no measurable effect against the same compound. Therefore, the microfluidic system formed by hESC-derived arterial ECs is a sensitive platform for embryonic vascular toxicological assessment.

The inhibitory mechanism of 7-Cyclo against embryonic arterial cells involves the inhibition of tyrosine kinases highly expressed in embryonic ECs than in fetal or adult ECs. Results show that embryonic ECs (both human or mouse) express higher levels of tyrosine kinase genes that are susceptible to the inhibitory effect of 7-Cyclo. This might explain the enhanced susceptibility of hESC-derived ECs to the effect of 7-Cyclo. In conclusion, the platform described here is promising for the identification of compounds with embryonic toxicity as well as to study embryonic vascular biology under physiologic conditions.

Methods and Results

hESC Culture and Differentiation.

Undifferentiated hESCs (passages 33-36; H9, WiCell, Wisconsin) or hiPSCs K2 (passages 32-35; cord blood derived iPSCs kindly donated by Ulrich Martin) were grown on an inactivated mouse embryonic fibroblast (MEF) feeder layer, as previously described 1, 2. To induce the formation of EBs, the undifferentiated hESCs were treated with 2 mg/mL type IV collagenase (Invitrogen) for 2 h and then transferred (2:1) to low attachment plates (Corning) containing 10 mL of differentiation medium[80% KO-DMEM, 20% fetal bovine serum (FBS, Invitrogen), 0.5% L-glutamine, 0.2% β-mercaptoethanol, 1% nonessential amino acids]. The differentiation medium was supplemented with VEGF165 (50 ng/mL, Prepotech), Tβ4 (100 ng/mL, Caslo) and SB431542 (10 μM, Tocris) according to the following timeline: [(VEGF165)days0-18+(Tβ4)days4-18+(SB431542)days7-18]. After 4 days in suspension, EBs were plated onto 1% gelatin-coated dishes and grown for 14 additional days. Medium was changed every 2-3 days. To evaluate the contribution of Shh and Notch signaling pathways, the differentiation medium was supplemented with one or two of the following agents: 1 μM purmorphamine (Shh activator; Cayman Chemical), 5 μM cyclopamine (Shh inhibitor; Sigma), 100 ng/ml DLL-4 (Notch activator; Prepotech), or 1 μM Y-secretase inhibitor L685458 (Notch inhibitor; Tocris Biosciences) 47.

Isolation of CD31+ Cells.

CD31+ cells were isolated from differentiated hESCs at day 18 using MACS (Miltenyi Biotec). Isolated cells were grown on petri dishes (1.5×104 cells/cm2) coated with 0.1% gelatin and containing EGM-2 (Lonza) supplemented with SB431542 (10 μM). Cell characterization at gene, protein and functional levels can be found in Supplementary Information. HUVECS and HUAECs (both from Lonza) were used as controls for the differentiation studies. Cells were cultured in EGM-2 media or EGM-MV media (both from Lonza; until passage 5) and the medium changed every 2 days.

Immunofluorescence Analysis.

Cells were transferred to gelatin-coated slides containing differentiation medium, allowed to attach overnight, and then fixed with 4% paraformaldehyde (Electron Microscopy Sciences) for 15 min at room temperature, or cold methanol (5 min). In some cases cells were directly fixed in IBIDI slides. Cells were blocked with 1% (w/v) BSA and stained for 1 h with anti-human primary antibodies for CD34 (Dako; clone QBend10; 1:20), CD31 (Dako; clone JC70A; 1:20), vWf (Dako; clone F8/86; 1:50), VECad (Santa Cruz Biotech; clone F-8; 1:50), α-SMA (Dako; clone 1A4; 1:50), EphB2 (Santa Cruz Biotech; clone H-83; 1:50), Lefty (Santa Cruz Biotech; clone D-6m; 1:50), Podoplanin (Santa Cruz Biotech; clone E-1; 1:50) and HSPG (US Biological; clone 10E4; 1:100). In each immunofluorescence experiment, an isotype-matched IgG control was used. Binding of primary antibodies to specific cells was detected with anti-mouse IgG Cy3 conjugate (Sigma, 1:50), anti-rabbit Cy3 (Jackson Labs, 1:100) or anti-goat (alexa488, Molecular Probes, 1:200). If necessary, permeabilization with 0.1% TritonX-100 in PBS was performed. For phalloidin staining, cells were stained with 50 μg/mL FITC-phalloidin (Sigma). Cell nuclei were stained with 4′, 6′-diamidino-2-phenylindole (DAPI) (Sigma) and the slides examined with either a Zeiss fluorescence microscope or Zeiss LSM 50 confocal microscope. For uptake of Dil-labeled acetylated low-density lipoprotein (Dil-Ac-LDL), cells were incubated with Dil-labeled Ac-LDL (10 μg/mL, Biomedical Technologies) for 4 h at 37° C. After incubation, cells were washed three times in EGM-2, fixed with 4% (w/v) paraformaldehyde for 30 min and visualized in a fluorescent microscope.

Gene and Intracellular Ca²⁺ Analyses.

A detailed methodology for both analyses can be found in the online supplementary information. Preferably, HUVEC, HUAEC or hESC-derived ECs were loaded with Fura-2 calcium fluorescent indicator by incubation with 5 μM of the membrane permeable acetoxymethyl (AM) derivative FURA-2/AM (1 mM in DMSO, Molecular Probes) and 0.06% (w/v) Pluronic F-127 (Sigma), using basal medium (M199, Sigma) as a vehicle (35 μL/well, not supplemented with serum nor antibiotics), for 1 h at 37° C. in 5% CO₂ and 90% humidity. Cells were then stimulated with histamine (100 μM, Sigma), VEGF₁₆₅ (100 ng, Prepotech), prostaglandin H2-analogue U46619 (10 μM, Cayman) or thrombin (2U, Sigma). A detailed methodology for the fluorescence acquisition can be found on the online data supplement.

Fluidic Shear Stress Experiments.

HESC-derived ECs (2.6×10⁶ cells/mL), HUAECs, HUVECs were seeded on flow chamber untreated-slides (p-slide 0.4, luer, IBIDI) and grown until confluence. Using a programmed IBIDI pump with positive pressure, the cell monolayer was perfused with EGM-2 medium for 7 days, at flow rate of 15.1 mL/min (corresponding to shear stress of 20 dyne/cm2), or 3.02 mL/min (corresponding to shear stress of 4 dyne/cm2). After 7 days the flow was stopped, the cells imaged, and then stained or collected for posterior gene analysis. Medium was collected for ELISA assays. In some experiments, cells were cultured in medium supplemented with terbinafine (0.1 μM or 1 μM) (Sigma) for a maximum of 24 h, under shear stress. Static controls were performed in the chamber coated-slides but without flow. The IBIDI slides allow a laminar perfusion in rectangular flow geometry.

For the 7-cyclo studies the flow experiments were carried out in a microfluidic system developed by us. The microfluidic device was obtained using a cylindrical mold of 20 Gauge. Polydimethylsiloxane (PDMS) (Sylgard 184 Silicone elastomere base) was mixed at a 10:1 ratio (w/w) with curing agent (Sylgard 184 Silicon Elastomer Curing Agent) and the solution was poured onto a mold (20 gauge needle laid on a plastic dish) and cured at 80° C. for about 3 h. The microchannel was cut to 0.5 cm length (900 μm inner diameter) and treated with Plasma Clean (Electronic Diener Femto Plasma Surface Technology version 5) for 2 min with argon gas (2 mBar) before immersing it in a solution of 0.1% gelatin. HESC-derived ECs (20×10⁶ cells/mL) or HUAECs were seeded on PDMS microchannels (3.18 μL cell suspension per channel) devices and grown until confluence. Using a programmed IBIDI pump with positive pressure, the cell monolayer was perfused with EGM-2 medium for 7 days; shear stress of 20 dyne/cm2 (After 7 days the flow was stopped, the cells imaged, and then stained or collected for posterior gene analysis. Medium was collected for ELISA assays. In some experiments, cells were cultured in medium supplemented with 1 μM 7-cyclo (Sigma) for an additional time of 24 h, under shear stress. Static controls were carried out in the microfluidic system but without flow.

Evaluation of the Levels of Vascular Injury by Specific Markers.

ELISA kits analyzed supernatants collected from the shear stress experiments for vWF and vWFpp (Gen-Probe GTI Diagnostic) and ADMA (Enzo Life Sciences), according to manufacturer's recommendations.

High-Throughput Screening (HTS).

HUAECS (Lonza) and hESC-derived ECs were cultured in EGM-2 medium while human anterior cruciate ligament cells (ACL cells) were cultured in Dulbecco's modified Eagle's medium (DMEM; PAA) supplemented with 10% FBS, 0.2 mM ascorbic acid 2-phosphate magnesium salt (Sigma Aldrich), 100 μM/mL streptomycin and 100 U/mL penicillin (Life Technologies). ACL cells were isolated from patients that have signed an informed consent form, in compliance with the Dutch legislation. The ethical committee of Medisch Spectrum Twente Hospital approved the collection.

The LOPAC library (Sigma-Aldrich) was used to screen embryonic endothelial-specific cytotoxic compounds. The compounds were solubilized in DMSO. The final compound concentration used in the screen was 4.5 μM in a final volume of 200 μL per well (96-well plate). HUAECs, hESC-derived EC and ACL cells were seeded at 16,000 cells/well (HUAECs and hESC-derived ECs) and 5,000 cells/well (ACL cells) and allowed to reach near confluence (approximately two days). After two days, medium was exchanged and test compounds and controls were added to the 96-well plates (all wells contained 0.25% (v/v) DMSO). After 4 days of incubation, cell viability was assessed using a PrestoBlue (Invitrogen) assay. This assay is based on a resazurin-based solution that indicates the reducing power of living cells and therefore measures indirectly their number. For each measurement, cell medium was removed and the PrestoBlue solution (10%) was added for 3 h at 37° C. upon which the absorbance was measured at 560-590 nm. Initially a list of compounds were selected based on their higher cytotoxic to HUAECs (more than 50%) than ACL cells. Then, a list of compounds was selected based on their higher cytotoxicity to hESC-derived ECs (more than 20%) than HUAECs.

The hit compounds obtained from the primary screen were then re-evaluated at eight different concentrations in order to find a dose-response curve. hESC-derived ECs and HUAECs were seeded at 16,000 cells/well (96 well plate), in EGM-2, and allowed to reach sub-confluency. The compounds were serially diluted in DMSO in logarithmic steps, ranging from 0.01 μM to 100 μM and added to the cell culture medium (200 μL per well; EGM-2 medium). Untreated cells were used as control. Cell viability was assessed as described before for the LOPAC library using the PrestoBlue assay.

Statistical Analysis.

An unpaired t test or one-way ANOVA analysis of variance with Newman-Keuls post-test was performed for statistical tests using software GraphPad Prism™. Results were considered significant when P<0.05.

Results:

Derivation of Arterial ECs from hPSCs

To differentiate hPSCs into arterial ECs several protocols using VEGF₁₆₅ ^(1, 17), thymosin β4 (Tβ4) were screened^(18, 19) and TGF-β inhibitor (SB431542)²⁰ as inductive agents of EC differentiation (FIGS. 7A and 8A). To determine their inductive effect the expression of CD31 marker was examined, which has been previously used to identify embryonic ECs′, by flow cytometry (FIGS. 7B and 8B). Using the inductive protocol (Prot 7) was possible to obtain approximately 5% of CD31⁺ at the end of 18 days of differentiation. This percentage is two fold higher (P<0.05, n=3) than the one obtained for the spontaneous differentiation of the cells (Prot 1) using classical EB medium (≈2%). The phenotypic analyses were complemented by gene expression analyses by qRT-PCR (FIGS. 7C and 8C).

To determine whether CD31⁺ cells could give rise to ECs, CD31⁺ cells were isolated by magnetic activated cell sorting (MACS) and cultured in EGM-2 medium supplemented with SB431542 (10 μM) (FIG. 9A). Gene expression analysis in cells differentiated for 3 passages (between 18 and 22 days after cell seeding) indicate that they express CD34, VE-Cad and Flk-1/KDR at the same or higher level as the one found in HUVECs, albeit have lower expression of vWF and CD31 which may indicate different levels of maturation (FIG. 7D). Furthermore, the differentiated cells had low expression of Oct-4 confirming their differentiated state. Flow cytometry and immunocytochemistry analyses show that CD31+ cells cultured for 3 passages expressed high levels of EC markers (FIG. 7E and FIG. 7A). Differentiated CD31⁺ cells stained positively for VE-Cad at cell-cell adherent junctions and produced vWF (FIG. 1A). In addition, they do not express markers of other mesoderm-derived cell lineages such as the smooth muscle cell marker α-SMA (FIG. 9B). Overall, the results indicate that CD31⁺ cells give rise to ECs.

Next, the sub-phenotype of hESC-derived ECs was determined, i.e., whether the ECs have been committed to arterial, venous or lymphatic lineages. Was used podoplanin (podocyte membrane mucroprotein21), EphB2 (a transmembrane ligand13) and Lefty 1/222 as lymphatic, arterial and venous markers, respectively. As expected, human umbilical arterial ECs (HUAECs) express EphB2 but do not express Lefty 1/2 while HUVECs express Lefty 1/2 but do not express EphB2 (FIG. 9C). HESC-derived ECs stain positive for arterial marker EphB2 but not for the venous marker Lefty 1/2 or the lymphatic marker podoplanin (FIG. 1A). Gene microarrays were also performed to confirm arterial sub-phenotype. HESC-derived ECs express most of arterial markers shown by HUAECs or human aortic arterial endothelial cells (HAECs) such as JAG1, JAG2, EFNB1, EFNB2, NOTCH1, NOTCH2, NOTCH3, NOTCH4, DLL1, DLL3, DLL4, ALDHA1, HEY2, LIPG, CD44, KRT78, FSTI)²² (FIG. 1B and Table 1). These results were further confirmed by qRT-PCR. hESC-derived ECs have high expression of arterial genes such as JAG1, EFNB1 and Hey-2 and low expression of venous genes such as EphB4, Lefty-1 and Lefty-2 (FIG. 1C). Together, the results indicate that hESC-derived ECs have been coaxed into arterial ECs. The strategy was initially tested with H9 hESC line; however, the protocol robustness was subsequently validated on a hiPSC line (K2) generated from cord blood²³ (FIG. 10).

Next, we asked whether the ECs have been committed to arterial phenotype at the isolation of CD31⁺ cells or after their differentiation. Both arterial and venous genes are expressed during the inductive and spontaneous differentiation protocols (FIG. 5A), and the inductive protocol does not seem to increase the expression of arterial genes as compared to the spontaneous protocol (FIGS. 11B and 11C).

Arterial and venous gene expression was analyzed in CD31⁺ cells at the time of their isolation. Both venous and arterial markers were expressed at similar magnitude (FIG. 1D, orange columns), which suggests that sub-phenotype is not defined at the isolation of CD31⁺ cells but only after their differentiation. Because arterial/vein specification during mouse embryogenesis is in large part mediated by Notch and Sonic Hedgehog (Shh), the expression of Notch (Notch1, Notch2, Notch3, Notch4) and Shh (patched 1) receptors as well as Notch (DLL1, DLL3, DLL4, JAG-1, JAG-2) and Shh transcription factors (GLI1 and SMO) in CD31⁺ cells was assessed (FIG. 1E). The results show that CD31⁺ cells after isolation express both Notch and Shh ligands and receptors. Was also studied the involvement of Notch and Shh signaling pathways in the gene expression of arterial and venous markers in cells after CD31⁺ cell isolation. The independent activation of each signaling pathway has little impact in the expression of arterial marker EphB2 (FIG. 1F). However, the simultaneous activation of Shh and Notch signaling by purmorphamine and DLL4, respectively, significantly increased the expression of arterial EC markers such as EphB2 (protein level), HEY-2, JAG-1 and EFNB1 (gene level), and simultaneously decreased the expression of venous EC markers such as Lefty-1 and Lefty-2 (FIG. 1D), as compared to cells cultured without these activators. By other hand, inhibiting both Shh and Notch signaling by cyclopamine and an inhibitor of μ-secretase, respectively, significantly decreased the expression of arterial marker EphB2 (FIG. 1F). Taken together, these results indicate that arterial specification mainly occurs after the isolation of CD31⁺ cells, and Shh and Notch signaling pathways mediate at least in part the specification of hPSCs into the arterial phenotype.

To determine whether the cells had an embryonic or adult arterial phenotype gene microarrays for hESC-derived ECs, HUAECs (fetal phenotype) and HAECs (adult phenotype) were performed. Comparing the expression of hESC-derived-EC and HUAEC we found a 67 up-regulated genes and 141 down-regulated genes (P<0.001 and cutoff of three times up- or down-regulated). Embryonic gene candidates were identified from gene expression on ECs isolated from mouse at day E12.5 and P1 (after birth). The receptor protein tyrosine phosphatase μ (PTPRu), the T-cell acute lymphocyte leukemia 1 (TAL1), N-cadherin (CDH2), angiopoietin 1 (ANGPT1) and DNA-binding protein inhibitor ID-1 (ID1), are up-regulated in mouse ECs E12.5 as compared to mouse ECs P1 (FIG. 1G). Similarly, those genes are up-regulated in hESC-derived ECs as compared to HUAECs. The receptor protein tyrosine phosphatase ρ (PTPRu) has been reported as present in the aorta of mouse embryos, but not in the adult mice²⁴. The T-cell acute lymphocyte leukemia 1 (Tali), also known as ScI, is a transcription factor (basic helix-loop-helix) several times associated with newly formed vessels and absent in quiescent endothelium^(25, 26). In addition, major cadherins expressed in early embryos of Xenopus laevis are E-cadherin, N-cadherin and a maternal cadherin known as either C-cadherin or EP-cadherin^(27,28). Microarray analyses showed also that the previous embryonic genes as well as others are higher expressed in hESC-derived ECs than in hUAECs or hAECs (FIG. 1H and Table 2). Overall, results indicate that the hESC-derived ECs have an embryonic arterial phenotype.

To determine whether the ECs were functional was evaluated their ability to uptake Dil-labeled acetylated low-density lipoprotein (Dil-Ac-LDL), to form cord-like structures and to respond to vasoactive agonists. FIG. 1A confirm that hESC-derived ECs are able to take up Dil-Ac-LDL and to form cord-like structures when cultured in the basement membrane Matrigel. Furthermore, the hESC-derived ECs respond to the vasoactive agonists as normal ECs by increasing the intracellular levels of Ca²⁺ (FIG. 11). Cells were loaded with FURA-2, a Ca²⁺ sensitive dye, and their response to known concentrations of vasoactive agonists histamine, VEGF₁₆₅, prostaglandin H2-analogue U46619 and thrombin was monitored by fluorescence. The response profile was compared to the one observed for HUVECs and HUAECs. HESC-derived ECs share a similar response profile to thrombin as HUAECs but different response profiles to VEGF₁₆₅, prostaglandin H2-analogue and histamine. No similarity was found in the response profiles of hESC-derived ECs and HUVECs. Together, results show that hESC-derived ECs are functional however showing different response profiles to vasoactive agonists as compared to HUAECs and HUVECs. The differences found between hESC-derived ECs and somatic arterial ECs (i.e. HUAECs) are likely ascribed to their embryonic and adult phenotypes.

Influence of Shear Stress in EC Morphology, Modulation of Arterial Sub-Phenotype and Glycocalix Expression

Having demonstrated the differentiation of hESCs into arterial ECs was studied how the differentiated cells responded to arterial (20 dyne/cm²) and venous (4 dyne/cm²) shear stress²⁹. It has been shown that a mechanosensory complex formed by CD31, VE-Cad and VEGFR-2 mediates the responsiveness of ECs to shear stress¹⁶ ³⁰. The activation of this complex leads to integrin activation and alignment which triggers the activation of VEGFR-2 tyrosine kinases, extracellular signal-regulated kinases (ERKs), c-Jun amino-terminal kinases (JNKs), p38 mitogen-activated protein kinase and AKT serine/thronine kinases and transcription factors such as NF-kB. To study the influence of shear stress, hESC-derived ECs (or HUAECs as control) were cultured for 7 days in flow conditions. HESC-derived ECs cultured in arterial flow (20 dyne/cm²) conditions align morphologically in the direction of the flow and show alignment of the proteins VE-Cad and actin (stained with phalloidin) (FIG. 2A). Similar results have been obtained for HUAECs (FIG. 12). The alignment of the cells is dependent on the magnitude of the shear stress since cells cultured in low (4 dyne/cm²) or no flow (static conditions) show low or no alignment, respectively. Importantly, cells cultured in flow or static conditions express EphB2 but not lefty A/B (FIG. 2A and FIG. 12). The expression of EphB2 is higher in flow than in static conditions, since 75% and 50% of the cells express EphB2 in flow and in static conditions, respectively (FIG. 2B). In addition, the expression of CD31, VE-Cad and VEGFR-2 (KDR/FIK-1) is also up-regulated in flow conditions (FIG. 2B). The up-regulation of these proteins is in agreement with previous data showing that their involvement on a mechanosensory complex that mediates the EC response to fluid shear stress³⁰. The maturation of hESC-derived ECs into arterial cells was also observed at gene level (FIG. 2C). Cells cultured under arterial or venous flow conditions express higher levels of arterial genes such as Notch receptors (1, 2, 3 and 4), Notch ligands (DLL3 and Jagged-1), Notch transcription factor Hey-2, aldehyde dehydrogenase 1 (ALDH1A1), and ephrin-B1 (EFNB1) and ephrin-B2 (EFNB2), which are ligands of ephrin receptors. Gene expression increased with an increase with flow. An upregulation of venous genes such as EphB4 and Lefty was also observed when the cells were cultured under flow conditions; however, no lefty protein was observed under these conditions (FIG. 2C and FIG. 12A). Interestingly, HUAECs cultured under static or flow conditions have the same gene expression profile (FIG. 12B), and therefore it suggests that hESC-derived ECs are more prone to respond to shear stress than mature somatic cells. Overall, the results indicate that hESC-derived ECs respond to arterial flow by the alignment of VECAD and actin fibers, up-regulation of the mechanosensory complex VECad, CD31 and VEGFR2, and by the up-regulation of arterial marker EphB2.

Heparan sulfate proteoglycan (HSPG), a component of glycocalyx layer of ECs, has been reported to be a fluid stress sensor on ECs^(31, 32). A class of HSPG, syndecans, is known to associate with cytoskeletal elements including actin, either directly or through associated actin-binding proteins³¹. Importantly, HSPG is absent on ECs grown and maintained under standard cell culture conditions in vitro, i.e. without flow³³. Therefore was investigated whether hESC-derived ECs express HSPG under flow culture conditions (20 dyne/cm²). Confocal microscopy analysis show that the surface of hESC-derived ECs or control HUAECs cultured under flow conditions is abundantly decorated with HSPG while no expression of HSPG is observed in static conditions (FIG. 2D). The HSPG is detected in the apical region of ECs (XZ view) exposed to flow. These results show that hESC-derived ECs can respond to flow by producing HSPG, as it is observed in vivo conditions.

Vascular Toxicity Assessed in hESC-Derived ECs Cultured Under Flow Conditions

Terbinafine is an antifungal agent (inhibitor of ergosterol synthesis) that inhibits angiogenesis by suppressing endothelial cell proliferation, inhibits DNA synthesis and activates EC apoptosis^(34, 35). Terbinafine is cytotoxic for HUVECs for concentrations above 120 μM³⁶. To determine whether the hESC-derived ECs cultured under flow conditions could be used to assess vascular toxicity, cells were cultured for 7 days at 20 dyne/cm2 after which the culture medium was supplemented or not with terbinafine (0.1 and 1 μM) and cells cultured under the same flow conditions for one more day. No significant changes in terms of EC morphology and cadherin expression were observed in ECs cultured in the presence or absence of the drug at day 8 (FIG. 3A). Next, was assessed by qRT-PCR the expression of genes involved in inflammation (ICAM-1; E-selectin), oxidative stress sensing (HO-1) and vasculature modulation (eNOS) in hESC-derived cells and control HUAEC after exposition to terbinafine (FIG. 3B). As expected, the expression of these genes increases under fluidic shear stress³⁶. Interestingly, the expression of the genes is higher when hESC-derived ECs were cultured in the presence of terbinafine at 0.1 μM. Upregulation of some genes (HO-1 and E-selectin) was only observed for concentrations of 1 μM in HUAECs. These results were complemented by evaluating the expression of two genes related to the expression of dimethylarginine-dimethyl-amino-hydrolases (DDAH), a family of enzymes that metabolizes asymmetric dimethylarginine (ADMA)³⁷. ADMA is a naturally occurring amino acid that circulates in plasma and is generated by degradation of methylated proteins. Increase levels of ADMA inhibits nitric oxide (NO) production by competitive inhibition of the physiological substrate L-arginine³⁸. Drugs that inhibit NO pathway, upregulate caveolin-1 expression and/or enhance ADMA levels could potentially be associated with drug-induced endothelial dysfunction. Therefore, increased circulating levels of ADMA may serve as predictive marker of EC dysfunction³⁸. To date, two isoforms of DDAH (DDAH1 and DDAH2) have been found in mammals. hESC-derived ECs cultured under flow conditions in medium supplemented with terbinafine show a significant decrease in the expression of DDAH1 and DDAH2 (P<0.05 or P<0.01, n=4). Such effect was not observed for HUAECs cultured under the same conditions. Interestingly, the decrease of DDAH1 and DDAH2 in hESC-derived ECs was only observed when cells were culture under flow conditions. Overall results indicate that cells cultured under physiologic shear stress have higher sensitivity to cytotoxic compounds such as terbinafine than cells cultured in static conditions. In addition, hESC-derived ECs are more sensitive to the effect of terbinafine than HUAECs.

Next, the vascular damage/injury induced by terbinafine was assessed in hESC-derived ECs and control HUAECs, in the same conditions as before, by measuring the levels of ADMA and the ratio von Willebrand factor pro-peptide (vWFpp): von Willebrand factor (vWF) secreted by these cells (FIG. 3C). The last 2 proteins with different biological activity are released in equimolar concentrations in plasma and an increased level of both proteins predicts EC activation/injury. Analysis of vWFpp:vWF ratio may discriminate and/or differentiate acute from chronic and progressive vascular injury39, 40. hESC-derived ECs or HUAECs cultured in static conditions in the presence of the drug have no significant higher secretion of ADMA or vWFpp:vWF than in control conditions (i.e., without the drug). Remarkably, hESC-derived ECs cultured under flow conditions in the presence of terbinafine secrete higher levels of ADMA and vWFpp:vWF than without the drug. Again, the sensitivity of HUAECs to terbinafine was lower than hESC-derived ECs. The concentration of ADMA secreted by hESC-derived cells at 24 h is 3.5 times higher than in basal conditions. Importantly, this shift in concentration is observed in human patients with cardiovascular diseases. Healthy persons have an average of 0.45±0.19 μmol of ADMA per liter of blood and this value increases 2.2 or 2.7 folds when the patient has idiopathic pulmonary arterial hypertension³⁷. Furthermore, the secreted concentrations of ADMA correlate with the concentration of terbinafine in the culture medium. The quantification of vWFpp:vWF ratio confirm some of the results obtained by ADMA. Again, no effect of terbinafine was observed in hESC-derived ECs or HUAECs cultured in static conditions. In contrast, both cells cultured in flow conditions and exposed to terbinafine secrete higher levels of vWFpp:vWF, confirming vascular dysfunction.

Identification of Embryonic Arterial Cell Inhibitors by High-Throughput Screening Followed by the Test of the Hits in Flow Cell Culture Conditions.

Disruption of vascular development has been directly correlated with prenatal loss, malformations and neurodevelopmental problems⁴¹⁻⁴⁴. To study embryonic vascular toxicity, embryonic arterial ECs were exposed to a Library of Pharmacologically Active Compounds (LOPAC, Sigma-Aldrich) comprising 1280 bioactive compounds, and we assessed cell viability after 4 days by a PrestoBlue assay (resazurin-based solution that is reduced by viable cells) (FIG. 4A). The library was screened at a single dosage of 4.5 μM in a volume of 200 μL per well of EGM-2 medium, containing 0.25% DMSO (v/v). To identify compounds that selectively target endothelial cells, the same library against human anterior cruciate ligament cells (ACL cells) was screened. From the 1280 compounds, 99 compounds induced differences in cell viability (hESC-derived ECs versus ACL cells) above 50% and therefore were further considered for the next step (Table 4). To identify compounds that are selective to embryonic ECs but not fetal ECs were screened the library against HUAECs (FIG. 4A). Six compounds (danazol, chlorpromazine hydrochloride, ellipticine, 3′,4′-dichlorobenzamil, fluphenazine dihydrochloride and 7-cyclopentyl-5-(4-phenoxyl)phenyl-7H-pyrrolo[2,3-d]pyrimidin-4-ylamine) affected cell viability in both cells by a difference of 20% (FIG. 4B). The six compounds belong to different classes: cell cycle, ion pumps, dopamine pathway, phosphorylation and hormone signaling. The compounds selected from the primary screen were then tested against hESC-derived ECs and HUAECs at eight different concentrations to obtain a dose-response curve (FIG. 4C and FIG. 14). The compound 7-cyclopentyl-5-(4-phenoxyl)phenyl-7H-pyrrolo[2,3-d]pyrimidin-4-ylamine (belonging to the phosphorylation class of the library and abbreviated from now on as 7-Cyclo) was chosen for further testing due to the significant effects observed between hESC-derived ECs and HUAECs.

To test the 7-Cyclo properties in the disruption of vascular networks, microvessels of hESC-derived ECs, HUAECs and HAECs were formed on top of Matrigel and then exposed to the drug for 20 h. Results show that there is a statistically significant reduction in the network length and number of sprouts in microvessels formed by hESC-derived ECs after incubation with 1 μM of 7-Cyclo (FIG. 15) while negligible effect was observed in microvessels formed by HUAECs and HAECs.

To evaluate the effects of 7-Cyclo in flow conditions, hESC-derived ECs were cultured in a poly(dimethylsiloxane) (PDMS) microfluidic system with cylindrical channels for 7 days at 20 dyne/cm2 (FIG. 5A). ECs were able to form a confluent monolayer in the entire inner surface of the channel after 48 h. At day 7, cells were exposed to EGM-2 medium supplemented with 1 μM of 7-Cyclo for 24 h, and finally analyzed at morphological, genetic and secretion levels. Results show that hESC-derived ECs show significant alterations in cell morphology in contrast to HUAECs cultured under the same conditions (FIG. 5B). In addition, hESC-derived ECs exposed to 7-Cyclo express statistically higher levels of inflammatory genes, such as ICAM-1, E-selectin, HO-1 and eNOS (P<0.0001, n=4), and express statistically lower levels of DDAH1 and DDAH2 (P<0.05 or P<0.0001, n=4), enzymes that metabolize ADMA, than cells cultured under static conditions (FIG. 5C). In case of HUAECs cultured under the same conditions, the effect was less pronounced and only ICAM-1 and eNOS genes were up-regulated. No down-regulation of DDAH-1 and DDAH-2 was observed. These gene analyses were complemented by analyses of ADMA and the ratio vWFpp: von vWF secreted by these cells (FIG. 5D). hESC-derived ECs or HUAECs cultured in static conditions in the presence of the drug have similar secretion of ADMA or vWFpp:vWF as in control conditions (i.e., without the drug). Importantly, hESC-derived ECs cultured under flow conditions in the presence of 7-Cyclo secrete higher levels of ADMA (2.5 fold) and vWFpp:vWF (1.6 fold) than without the drug, and significant higher levels of ADMA than HUAECs. Overall, results indicate that hESC-derived ECs are more sensitive to the effect of 7-Cyclo than HUAECs showing high levels of vascular dysfunction.

To further validate the effects of 7-Cyclo at the embryonic vasculature, mouse embryonic ECs were incubated at day 12.5 (mAEC E12.5) and postnatal day 1 (p1) with 7-Cyclo (1 μM) for 24 h. Inflammation, oxidative stress sensing, vascular modulation and vascular injury sensing genes are statistically up-regulated in mouse aortic endothelial cells (mAEC) at E12.5 as compared to cells without treatment (FIG. 6A). In contrast, 7-Cyclo has no effect in p1 ECs. The degree of action of 7-Cyclo in mAEC E12.5 is similar to the one identified in hESC-derived ECs (FIG. 5C).

Mechanism of 7-Cyclo in Embryonic ECs

7-Cyclo is a cell-permeable pyrrolopyrimidine that acts as a potent inhibitor of tyrosine kinases such as SRC, KDR, TIE-2, BLK, FYN, LYN, CSK, EGFR, PKC, CDC2/B and ZAP-7045. To understand the distinctive effect of 7-Cyclo in embryonic versus fetal/adult ECs gene microarray data was analyzed and compared the expression of tyrosine kinases. From a group of 38 tyrosine kinases (FIG. 6B and Table 4), which includes some of the most (EFS, KDR, LYN, LKC) and less affected (EGFR, ZAP70) tyrosine kinases by 7-Cyclo in human cells (FIG. 6B, left panel), 8 of them (KDR, LYN, FLT1, FLT4, MERTK, NRP1, TEK, TYRO3) were higher expressed in hESC-derived ECs than in HUAECs or HEACs. In fact, some tyrosine kinases were only expressed in hESC-derived EC (EFS, EGFR, ZAP70, LTK, NTRK2). The expression of the most significant genes (EFS, KDR, LYN, LCK, EGFR, ZAP70, NRP1 and TEK) was further characterized and confirmed by qRT-PCR (FIG. 6C). The qRT-PCR included the analysis of mouse embryonic and postnatal ECs, to validate the results obtained for hESC-derived ECs. The same trend was observed, i.e, tyrosine kinases were found to be more expressed in embryonic ECs (both in human and mouse) than in fetal/adult tissues (exception for EGFR in mouse). Interestingly the expression of LCK, one of the targets of 7-Cyclo 46, was expressed at similar magnitude in hESC-derived ECs and HUAECs, as assessed by qRT-PCR, and thus the differences observed between embryonic and fetal/adult ECs are likely mediated by other tyrosine kinases described above. The kinase activity of the hESC-derived ECs and HUAECs was assessed by luminescence (signal is inversely correlated with the level of kinase activity) in the absence or presence of the compound 7-Cyclo (FIG. 6D). After 24 h incubation with 7-Cyclo (0.1 or 1 μM) the kinase activity of hESC-derived ECs decreased significantly from time zero (P<0.01), while no significant decrease was observed in HUAECs. Overall results indicate that 7-Cyclo affects hESC-derived ECs likely inhibiting tyrosine kinases highly expressed in the embryonic state such as KDR, LYN, FLT1, FLT4, MERTK, NRP1, TEK or TYRO3.

Expanded Materials and Methods:

hESC Culture and Differentiation._(—)

Undifferentiated hESCs (passages 33-36; H9, WiCell, Wisconsin) or hiPSCs K2 (passages 32-35; cord blood derived iPSCs kindly donated by Ulrich Martin) were grown on an inactivated mouse embryonic fibroblast (MEF) feeder layer, as previously described 1, 2. To induce the formation of EBs, the undifferentiated hESCs were treated with 2 mg/mL type IV collagenase (Invitrogen) for 2 h and then transferred (2:1) to low attachment plates (Corning) containing 10 mL of differentiation medium[80% KO-DMEM, 20% fetal bovine serum (FBS, Invitrogen), 0.5% L-glutamine, 0.2% β-mercaptoethanol, 1% nonessential amino acids]. The differentiation medium was supplemented with VEGF165 (50 ng/mL, Prepotech), Tβ4 (100 ng/mL, Caslo) and SB431542 (10 μM, Tocris) according to different timelines (see below). After 4 days in suspension, EBs were plated onto 1% gelatin-coated dishes and grown for 14 additional days. Medium was changed every 2-3 days. The experimental conditions tested were: (i) EB medium only (Prot1), (ii) EB medium supplemented with (ii) (VEGF165)days0-18 (Prot2), or (iii) [(VEGF165)days0-18+(SB431542, 10 μM)days7-18] (Prot3), or (iv) (VEGF165+Tβ4)days0-18 (Prot4), or (v) [(VEGF165)days0-18+(Tβ4)days4-18] (Prot5), or (vi) [(VEGF165+Tβ4)days0-18+(SB431542)days7-18] (Prot6), or (vii) [(VEGF165)days0-18+(Tβ4)days4-18+(SB431542)days7-18] (Prot7) (FIG. 7A). Protocol 7 gave the best results in terms of differentiation of hESCs into CD31+ cells.

Human and mouse primary cells. HUAEC (human umbilical arterial endothelial cells) and HUVEC (human umbilical venous endothelial cells) were bought from Lonza (http://www.lonza.com/). Total RNA of human Aortic Endothelial cells, mouse aortic endothelial cells (E 12.5 and p1) were bought from ScienceCell (http://www.sciencellonline.com/).

Flow Cytometry Analysis.

Cells were tripsinized, aliquoted (1.25-2.5×10⁵ cells per condition), washed in PBS, centrifuged at 1200 g and then resuspended in PBS containing 5% FBS. Cells were labeled with human CD31 monoclonal antibody (eBioscience, clone: WM59; 1.25:100), CD34 (Miltenyi Biotec, clone: AC136; 5:100), vwF (Dako; clone F8/86; 1:50), Flk-1/KDR (R&D, clone: 89106; 5:100), VeCad (R&D, clone: 123413; 5:100) or EphB2 (R&D, clone: 512012; 5:100). Cells were characterized on a FACS Calibur (BD) and the data analyzed by Cell Quest software. Twenty thousand events were collected in each run.

Quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) Analysis.

Total RNA from experimental groups was isolated using a protocol with TRIzol (Invitrogen) and RNeasy Minikit (Qiagen, Valencia). cDNA was prepared from 1 μg total RNA using Taqman Reverse transcription reagents (Applied Biosystems, CA). Quantitative PCR (qPCR) was performed using Power SYBR Green PCR Master Mix and the detection using an ABI PRISM 7500 Fast System (Applied Biosystems). Quantification of target genes was performed relative to the reference GAPDH gene: relative expression=2^([−(Ct) _(sample) ^(−Ct) _(GADPH) ^()]). The mean minimal cycle threshold values (Ct) were calculated from 4 reactions. In some cases, gene expression in each experimental group was normalized to the relative gene expression found in HUVECs, HUAECs or undifferentiated hESCs. Primer sequences are published in Table 1 of expanded materials and methods.

Matrigel Assay.

A 24-well plate was coated with Matrigel (0.4 mL, BD Biosciences) per well and incubated at 37° C. for 30 min. Cells were seeded on top of the polymerized Matrigel at a concentration of 1×10⁵ cells per 300 μL of EGM-2 medium. After 1 h of incubation at 37° C., an extra 1 mL of EGM-2 was added. After 12 h, medium supplemented with 7-Cyclo (0, 0.1 or 1 μM) was added. Cord formation was evaluated by phase contrast microscopy (Carl Zeiss International, Germany), at time 0, 3 h and 20 h after 7-Cyclo addition.

Intracellular Ca²⁺ variation measurements.

HUVECs, HUAECs or hESC-derived ECs were loaded with Fura-2 calcium fluorescent indicator by incubation with 5 μM of the membrane permeable acetoxymethyl (AM) derivative FURA-2/AM (1 mM in DMSO, Molecular Probes, http://www.invitrogen.com) and 0.06% (w/v) Pluronic F-127 (Sigma, http://www.sigmaaldrich.com), using basal medium (M199, Sigma) as vehicle (35 μl/well, not supplemented with serum nor antibiotics), for 1 h at 37° C. in 5% CO₂ and 90% humidity. The medium was then replaced by the respective basal medium and cells were incubated in the same conditions for 30 min to allow hydrolysis of the acetoxymethyl (AM) esters by cellular esterases, resulting in intracellular capture of the membrane impermeant Fura-2. Afterwards cells were washed twice with 100 μL sodium salt solution (140 mM NaCl, 5 mM KCl, 1 mM CaCl₂, 1 mM MgCl₂, 10 mM Glucose, 10 mM HEPES-Na⁺ pH 7.4). The buffer was replaced again (100 μl/well) immediately prior to incubating or not with test compounds.

Cells located in wells on a plate row were incubated at 25° C. (inside the microplate reader, during basal reading). Cells were then stimulated with 100 μM Histamine^(53,54) (Sigma), 100 ng VEGF₁₆₅ (Prepotech, www.peprotech.com)^(55,56), 10 μM Prostaglandin U46619 (Cayman, http://www.caymanchem.com)^(57,58), 50 mM KCl^(53,54) (Merck, http://www.merck.com) or 2U Thrombin (Sigma)⁵⁹ by adding 1 μl of a stock solution. Fluorescence was measured at emission 510 nm using two alternating excitation wavelengths (340 nm and 380 nm)⁶⁰ using a microplate fluorescence reader (Spectramax Gemini EM, Molecular Devices, with SoftMax® Pro software). The microplate reader was set to “top-read kinetics”; PMT was “high”; temperature was 25° C.; each fluorescence time point was an average of 18 reads; and during basal/inhibitors incubation periods and KCl/Histamine stimulation, each well was read every 3 sec. Fluorescence intensity values (in relative fluorescence units—RFU) measured at 340 nm and 380 nm were taken from the stabilized signal obtained at basal conditions and after incubation or not with the inhibitors. Following stimulation with Histamine/KCl, the fluorescence values were taken from the time point at the peak of the response. Typically, each experiment consisted of three or four wells plus three or four wells containing cells incubated with inhibitors and all cells were stimulated and read simultaneously. The dose-response curves for the effect of stimuli on the intracellular Ca²⁺ variation were determined using the software GraphPad Prism™.

Kinase Activity Quantification.

The kinase activity of cells was measured by a Kinase-Glo® Luminescence Kinase Assay (Promega). Cells incubated with 7-Cyclo for 24 h were treated with the kinase reagent from the kit and luminescence acquired using a Spectra Max Gemini luminometer. The luminescence signal is correlated with the amount of ATP present and its inversely correlated with the amount of kinase activity.

Shear Stress Calculations.

Using the Poiseuille's Law equation for laminar flow TS=(4ρQ)/TTR³), where TS is hemodynamic shear stress, μ is fluid viscosity, Q is flow and R radius; a laminar flow of 29 mL/min was used to apply 20 dyne/cm² shear stress on a 0.45 mm radius channel seeded with cells as described before.

Microarray Procedure and Data Analysis. RNA Isolation and Sample Labeling.

hESC-derived EC, HUAEC or AEC (adult aortic endothelial cells) were homogenized in Trizol reagent (Invitrogen) and total RNA was extracted by using the RNeasy Mini Kit (Qiagen, Valencia, USA), according to manufacturer's instructions. The quality of the RNA was assessed in the Agilent 2100 Bioanalyser (G2943CA) using the RNA 6000 Pico Kit (5067-1513). Before labeling, a mixture of ten in vitro synthesized RNAs were added to total RNA to allow for hybridization quality control and normalization of the microarrays. The RNA was amplified and labeled according to the One-Color Microarray-Based Gene Expression Analysis (Agilent). The efficiency of cRNA synyhesis and dye incorporation was measured using NanoDrop ND-1000 UV-VIS spectrophotometer and samples with neither a yield below 1.65 μg nor a specific activity (pmol Cy3 per ug cRNA) below 6 were not considered for hybridization.

Microarray Hybridization.

Labeled cRNA was hybridized to the Whole Human Genome (4×44K) Microarray (G4112F from Agilent Technologies). From each sample, 1.65 μg cyanine 3-labeled cRNA was adjusted to 41.8 μl with DNAse-free Water, mixed with 11 μl Agilent 10× blocking Agent and 2.2 μl Fragmentation Buffer and incubated at 60° C. for exactly 30 minutes to fragment RNA. To stop de fragmentation reaction 55 μl of 2×GEx Hybridization Buffer was added. After a short spin-down, the labelled cRNA mixture was applied to a microarray slide, assembled in a SureHyb Hybridization Chamber fitted with a gasket slide (Agilent), and incubated for 18H at 65° C. in a hybridization oven (G2545A, SHEL LAB—Agilent), with 10 rpm rotation speed. Slides were washed as described in the Agilent One-Color Microarray-Based Gene Expression Analysis protocol. Afterwards, the microarray and gasket slide were briefly disassembled inside a staining dish containing 250 ml of GE Wash Buffer 1 and the slides (up to 4 slides) were washed in fresh 250 ml of GE Wash Buffer 1 solution at room temperature, during 1 minute, with gentle agitation from a magnetic stirrer. A second wash step was carried out by immersing the slides in GE Wash Buffer 2 solution, previously warmed to 37° C., during 1 minute, also with gentle magnetic stirring. Finally, slides were dried by centrifugation at 800 rpm for 3 minutes.

Image Acquisition and Data Processing.

Microarrays were scanned in the Agilent B Scanner (G2565BA) using specific scanning protocols for gene expression microarrays and the format 4×44K. Agilent Feature Extraction Image Analysis Software (Version 10.7.3.1) was used to obtain fluorescence intensities from raw microarray image files.

Normalization and Analysis of DNA Microarray Data.

Analysis of raw data was performed using BRB-ArrayTools v3.4.0 developed by Dr. Richard Simon and BRB-ArrayTools Development Team⁵⁴. BRB-Array Tools incorporates the Bioconductor R functions and the R programming language required for raw data normalization within arrays⁵⁵. Each gene's measured intensity was median normalized to correct for differences in the labelling efficiency between samples. This analyzes provided a median normalized dataset that was subjected to statistical analysis and clustering using MeV software⁵⁶. Here, genes were identified as differentially expressed using the following criteria: (i) Test-design: between subjects (test vs control); (ii) Variance assumption: Welch approximation; (iii) P-value parameters: p-values based on t-distribution; alpha critical p-value=0.001; (iv) False discovery corrections: just alpha

Gene Expression Regulation Analysis.

The previous step provided a differentially expressed genes (DEGs) list for each strain that was used to calculate the M-value and Fold-change variation. It was considered as differentially expressed a variation equal or higher than 2× between conditions. Only genes with significance level below an alpha corrected p-value of 10⁻³ were considered as differentially expressed. The down- and up-regulated genes were analyzed using DAVID 6.7 (Database for Annotation, Visualization and Integrated Discovery, http://david.abcc.ncifcrf.gov/) web-accessible program to identify the altered cellular processes and functions. The microarray data has been deposited in NCBI's Gene Expression Omnibus database and is accessible through the GEO series accession number GSE51642.

TABLE 1 Primers used for Real Time PCR¹ Sense Antisense GAPDH ¹ SEQ ID AGCCACATCGCTCAGACACC SEQ ID GTACTCAGCGCCAGCATCG NO: 1 NO: 60 PCAM ¹ SEQ ID GCTGTTGGTGGAAGGAGTGC SEQ ID GAAGTTGGCTGGAGGTGCT NO: 2 NO: 61 C CD34 ¹ SEQ ID TGAAGCCTAGCCTGTCACCT SEQ ID CGCACAGCTGGAGGTCTTA NO: 3 NO: 62 T VECad ¹ SEQ ID ACGGGATGACCAAGTACAGC SEQ ID ACACACTTTGGGCTGGTAG NO: 4 NO: 63 G vWF ¹ SEQ ID ATGTTGTGGGAGATGTTTGC SEQ ID GCAGATAAGAGCTCAGCCT NO: 5 NO: 64 T Flk-1 ¹ SEQ ID CTGGCATGGTCTTCTGTGAA SEQ ID AATACCAGTGGATGTGATG NO: 6 GCA NO: 65 GCGG Oct-4 ¹¹ SEQ ID GTGGAGGAAGCTGACAACAA SEQ ID CTCCAGGTTGCCTCTCACT NO: 7 NO: 66 C EphB4 SEQ ID CTCAGTTCGGATCCTACC SEQ ID AATGTCACCCACTTCAGAT NO: 8 NO: 67 Lefty-1 SEQID CTGACAAGTTACCTCACCTA SEQ ID GACACATTGGGCTTTCTG NO: 9 NO: 68 Lefty-2 SEQ ID GAACTGAATTGCTGTGTTATA SEQ ID AACCAGAATCCAGGTATCC NO: 10 TG NO: 69 Notch1 SEQ ID ATCTGAAATAGGAAACAAGT SEQ ID ATAACCAACGAACAACTACA NO: 11 GAA NO: 70 TAA Notch2 SEQ ID AACATCTCATCCATGCTTTG SEQ ID ACAGTGGTACAGGTACTTC NO: 12 NO: 71 Notch3 SEQ ID CGCTCGTCAGTTCTTAGA SEQ ID AAGGAAGGAAGAGACAGAG NO: 13 NO: 72 Notch4 SEQ ID ATTGACACCCAGCTTCTT SEQ ID GAGGACAAGGGTCTTCAA NO: 14 NO: 73 DLL3 SEQ ID TGTCCGTGAAATGAATTGG SEQ ID AAGAGAAGATGGCAGGTAG NO: 15 NO: 74 DLL4 SEQ ID GGAGGTATAAGGCAGGAG SEQ ID AGGTGTGGAAGGGTATTG NO: 16 NO: 75 JAG1 SEQ ID GTCTCAAAGAAGCGATCAG SEQ ID ATATACTCCGCCGATTGG NO: 17 NO: 76 JAG2 SEQ ID ACAATGGAGTATTCTCGGAT SEQ ID CTGGTAACAAACGCTACG NO: 18 A NO: 77 EFNB1 SEQ ID CAACACTGTCAAGATGGC SEQ ID CTCTTCTCTTCCTGGTTCA NO: 19 NO: 78 EFNB2 SEQ ID CCACAGATAGGAGACAAATT SEQ ID AGTTGAGGAGAGGGGTAT NO: 20 G NO: 79 ALDH1A1 SEQ ID GAGTTTGTTCATCCAATCGTA SEQ ID GGTGAGTAGGACAGGTAAG NO: 21 NO: 80 Hey-2 SEQ ID ACAACTTCAGAAGTGCCT SEQ ID GACAAGAGAGAGGTGGAG NO: 22 NO: 81 ICAM-1 SEQ ID CAAGGCCTCAGTCAGTGTGA SEQ ID CCTCTGGCTTCGTCAGAAT NO: 23 NO: 82 C E-selectin SEQ ID AGCTTCCCATGGAACACAAC SEQ ID CTGGGCTCCCATTAGTTCAA NO: 24 NO: 83 eNOS SEQ ID GATGCTCCCAACTTGACCTT SEQ ID TAGGTCTTGGGGTTGTCAG NO: 25 GACCAT NO: 84 G HO-1 SEQ ID GAAAAGCACATCCAGGCAAT SEQ ID GCTGCCACATTAGGGTGTC NO: 26 NO: 85 T DDAH1 SEQ ID GGACAAATCAACGAGGTGCT SEQ ID TAGCGGTGGTCACTCATCT NO: 27 NO: 86 G DDAH2 SEQ ID CTGTTGTGGCAGGCAGCAG SEQ ID GTCAGGGAGGCATATGGGT NO: 28 NO: 87 G TALI SEQ ID ATGGAGATTACTGATGGTCC SEQ ID AG GATCTCATTCTTGCTGAG NO: 29 NO: 88 PTPRU SEQ ID ACATAGATGGTTACCACAGG SEQ ID AGAAGTCATAGACCATCTCA NO: 30 NO: 89 G CDH2 SEQ ID ACATATGTGATGACCGTAAC SEQ ID TTTTTCTCGATCAAGTCCAG NO: 31 NO: 90 ANGP1 SEQ ID ATGTTAACAGGAGGATGGTG SEQ ID GAAGTAGTGCCACTTTATCC NO: 32 NO: 91 ID1 SEQ ID ACTAGTCACCAGAGACTTTA SEQ ID AAATCTGAGAAGCACCAAA NO: 33 G NO: 92 C EFS SEQ ID ACCTCATCTACAAAATCCCC SEQ ID ACATCATAGGGAGCATCAT NO: 34 NO: 93 C KDR SEQ ID GTACATAGTTGTCGTTGTAG SEQ ID TCAATCCCCACATTTAGTTC NO: 35 G NO: 94 LYN SEQ ID CAACACCTTAGAAACAGAAG SEQ ID CTCTAATAAGGAAAGCTCCA NO: 36 AG NO: 95 G LCK SEQ ID ATGGGAGTCTAGTGGATTTT SEQ ID GGTCACGATGAATATAATTC NO: 37 C NO: 96 CG EGFR SEQ ID TCTTAAAGACCATCCAGGAG SEQ ID ATCTGCAGGTTTTCCAAAG NO: 38 NO: 97 ZAP70 SEQ ID CTGGATCTACAAGTGGGAG SEQ ID CCAGGCTGTAGTAACAGG NO: 39 NO: 98 NRP1 SEQ ID AGAAGATTGTGCAAAACCAG SEQ ID  TAAGGTCTTCAACACATTGC NO: 40 NO: 99 TEK SEQ ID GTGATTGACACTGGACATAA SEQ ID ACTTGAATATGTTGCCAAGC NO: 41 C NO: 100 mouseICAM- SEQ ID CAGTCTACAACTTTTCAGCTC SEQ ID CACACTTCACAGTTACTTGG 1 NO: 42 NO: 101 mouseE- SEQ ID CATGAAATGTCTTCCCAGTG SEQ ID ATCACATTTCACAGCTGAAC selectin NO: 43 NO: 102 mouseeNOS SEQ ID AAAGCTGCAGGTATTTGATG SEQ ID AGATTGCCTCTATTTGTTGC NO: 44 NO: 103 mouseHO-1 SEQ ID CATGAAGAACTTTCAGAAGG SEQ ID TAGATATGGTACAAGGAAG NO: 45 G NO: 104 CC mouseDDAH2 SEQ ID GACTCCCATCTTCATTAACTC SEQ ID TTCTCTTGACTTCACTGGTC NO: 46 NO: 105 mouseTAL1 SEQ ID AGATGGAGATTTCTGATGGT SEQ ID TTGCTTAGTTTCTTGTCTGG NO: 47 C NO: 106 mousePTPRU SEQ ID GGACATGATCTTTCTCAAGT SEQ ID TGGTAGCTGATCTCATACTG NO: 48 G NO: 107 mouseCDH2 SEQ ID GAGTTTACTGCCATGACTTTC SEQ ID TCCACCACTGATTCTGTATG NO: 49 NO: 108 mouseANGP1 SEQ ID CTGGAAGGAGTATAAAATGG SEQ ID TTCCTATGTGGAATCTGTCG NO: 50 G NO: 109 mouseID1 SEQ ID ATCTCTGGGAAAGACACTAC SEQ ID ATAAAACAGAAACACGCGG NO: 51 NO: 110 mouseEFS SEQ ID GGAATATGACTACGTGCATC SEQ ID TAGAAGTGCAGAAGTTGGA NO: 52 NO: 111 G mouseKDR SEQ ID GACGGATGATCAAGAGAAAT SEQ ID GTACCATTTGATATCAGGAG NO: 53 AG NO: 112 C mouseLYN SEQ ID GAAGCCATGGGATAAAGATG SEQ ID TGTTATAGTAACCCATCCAG NO: 54 NO: 113 AC mouseLCK SEQ ID GTGAAGAGTCTGAAACAAGG SEQ ID GGGAGTCTTGAGAAAATCT NO: 55 NO: 114 AC mouseEGFR SEQ ID CTGTCGCAAAGTTTGTAATG SEQ ID GAATTTCTAGTTCTCGTGGG NO: 56 NO: 115 mouseZAP70 SEQ ID CAGAAGCCCTACAAGAAAAT SEQ ID GTCACTCATAAGTGCATACA NO: 57 G NO: 116 TC mouseNRP1 SEQ ID TTATCTTTCAGGGAAACACC SEQ ID TCCAGAGCAAGGATAATCT NO: 58 NO: 117 G mouseTEK SEQ ID ATTTCCGTCAAAGTTCTTCC SEQ ID AAGCTTCTTGGATTTGATGG NO: 59 NO: 118 ¹PCR conditions: initial denaturation step at 94° C. for 5 min; 40 cycles of denaturation at 94° C. for 30 sec, annealing at 60° C. for 33 sec and extension at 72° C. for 30 sec. At the end was performed a final 7 minutes extension at 72° C.. After amplification, melting curves were acquired and used to determine the specificity of PCR products, which were further confirmed using conventional gel electrophoresis.

TABLE 2 Genes represented in heat map FIG. 1-B Probe NM pubmed Symbol Complete name A_23_P210763 NM_000214 JAG1 jagged 1 A_23_P106024 NM_002226 JAG2 jagged 2 A_24_P365807 NM_004429 EFNB1 ephrin-B1 A_23_P428139 NM_004093 EFNB2 ephrin-B2 A_23_P60387 NM_017617 NOTCH1 notch 1 A_23_P200792 NM_024408 NOTCH2 notch 2 A_24_P399606 NM_000435 NOTCH3 notch 3 A_23_P365614 NM_004557 NOTCH4 notch 4 A_23_P167920 NM_005618 DLL1 delta-like 1 (Drosophila) A_23_P16438 NM_016941 DLL3 delta-like 3 (Drosophila) A_23_P419641 NM_019074 DLL4 delta-like 4 (Drosophila) A_23_P83098 NM_000689 ALDH1A1 aldehyde dehydrogenase 1 family, member A1 A_24_P363408 NM_012259 HEY2 hairy/enhancer-of-split related with YRPW motif 2 A_23_P78405 NM_006033 LIPG lipase, endothelial A_23_P24870 NM_000610 CD44 CD44 molecule (Indian blood group) A_23_P331098 AK096419 KRT78 keratin 78 A_23_P110531 NM_013409 FST follistatin

TABLE 3 Genes represented in heat map FIG. 1-H Probe NM pubmed Symbol Complete name A_23_P63371 NM_003189 TAL1 T-cell acute lymphocytic leukemia 1 A_23_P149064 NM_005704 PTPRU protein tyrosine phosphatase, receptor type, U A_23_P137381 NM_002167 ID3 inhibitor of DNA binding 3, dominant negative helix- loop-helix protein A_23_P252306 NM_002165 ID1 inhibitor of DNA binding 1, dominant negative helix- loop-helix protein A_23_P216023 NM_001146 ANGPT1 angiopoietin 1 A_23_P60079 NM_001147 ANGPT2 angiopoietin 2 A_23_P132378 NM_014246 CELSR1 cadherin, EGF LAG seven- pass G-type receptor 1 (flamingo homolog, Drosophila) A_23_P144627 NM_018933 PCDHB13 protocadherin beta 13 A_23_P38732 NM_001792 CDH2 cadherin 2, type 1, N-cadherin (neuronal)

TABLE 4 List of the 99 that induced differences in cell viability (hESC-Derived ECs versus ACL cells Name Class Differences (%) S-(p-Azidophenacyl)glutathione Multi-Drug Resistance 61.3 AA-861 Leukotriene 53.3 5-azacytidine DNA Metabolism 56.1 Brefeldin A from Penicillium brefeldianum Cytoskeleton and ECM 95.5 Chlorprothixene hydrochloride Dopamine 58.1 (±)-Butaclamol hydrochloride Dopamine 53.6 BWB70C Leukotriene 55.3 5-Bromo-2′-deoxyuridine DNA Metabolism 53.9 Bromoacetyl alprenolol menthane Adrenoceptor 93.4 Cantharidin Phosphorylation 94.3 Chlorpromazine hydrochloride Dopamine 57.2 Calmidazolium chloride Intracellular Calcium 96.1 7-Chloro-4-hydroxy-2-phenyl-1,8-naphthyridine Adenosine 59.1 Colchicine Cytoskeleton and ECM 67.4 Calcimycin Intracellular Calcium 96.2 Cantharidic Acid Phosphorylation 93.8 Dequalinium dichloride K+ Channel 60.6 (S)-(+)-Camptothecin Apoptosis 96.3 10-(alpha-Diethylaminopropionyl)-phenothiazine Biochemistry 56.8 hydrochloride Dihydroouabain Ion Pump 97.0 Diphenyleneiodonium chloride Nitric Oxide 69.6 Dequalinium analog, C-14 linker Phosphorylation 96.6 3′,4′-Dichlorobenzamil Ion Pump 54.2 Etoposide Apoptosis 59.6 DCEBIO K+ Channel 73.2 Danazol Hormone 52.7 7-Cyclopentyl-5-(4-phenoxy)phenyl-7H-pyrrolo[2,3- Phosphorylation 65.4 d]pyrimidin-4-ylamine Emetine dihydrochloride hydrate Apoptosis 93.4 rac-2-Ethoxy-3-octadecanamido-1- Phosphorylation 56.2 propylphosphocholine R-(−)-Fluoxetine hydrochloride Serotonin 54.5 Felodipine Ca2+ Channel 58.1 Ergocristine Dopamine 52.5 Fluspirilene Dopamine 86.7 Felbamate Glutamate 50.5 Fiduxosin hydrochloride Adrenoceptor 58.2 cis-(Z)-Flupenthixol dihydrochloride Dopamine 69.6 Ellipticine Cell Cycle 58.5 5-fluoro-5′-deoxyuridine DNA Metabolism 60.8 S-(+)-Fluoxetine hydrochloride Serotonin 51.4 Fluphenazine dihydrochloride Dopamine 51.4 Idarubicin DNA Metabolism 94.0 Indirubin-3′-oxime Phosphorylation 85.5 SB 228357 Serotonin 62.7 LY-367,265 Serotonin 51.1 NNC 55-0396 Ca2+ Channel 98.4 Ivermectin Cholinergic 78.7 Metergoline Serotonin 56.5 Nocodazole Cytoskeleton and ECM 76.4 2-methoxyestradiol Hormone 84.3 Mibefradil dihydrochloride Ca2+ Channel 62.2 Mevastatin Antibiotic 98.0 Mitoxantrone DNA Metabolism 94.4 GR 127935 hydrochloride Serotonin 66.1 Methiothepin mesylate Serotonin 52.4 Niclosamide Antibiotic 97.6 1,3-Dimethyl-8-phenylxanthine Adenosine 68.8 Podophyllotoxin Cytoskeleton and ECM 75.7 Ouabain Ion Pump 94.6 SU 6656 Phosphorylation 61.5 Propionylpromazine hydrochloride Dopamine 50.3 S(−)-3PPP hydrochloride Dopamine 51.8 IC 261 Phosphorylation 73.0 Rottlerin Phosphorylation 61.6 N-Oleoyldopamine Neurotransmission 54.8 Risperidone Dopamine 56.9 SCH-202676 hydrobromide G protein 53.2 SR 2640 Leukotriene 53.0 SKF 96365 Ca2+ Channel 69.8 Sanguinarine chloride Ion Pump 96.4 Triflupromazine hydrochloride Dopamine 53.0 Tyrphostin AG 537 Phosphorylation 52.3 N-p-Tosyl-L-phenylalanine chloromethyl ketone Biochemistry 63.9 Trequinsin hydrochloride Cyclic Nucleotides 50.7 Tyrphostin AG 490 Phosphorylation 56.3 SB 224289 hydrochloride Serotonin 96.7 Tyrphostin AG 879 Phosphorylation 54.4 TTNPB Transcription 92.6 Tyrphostin AG 527 Phosphorylation 53.4 Taxol Cytoskeleton and ECM 78.1 Tomoxetine Adrenoceptor 68.7 Tamoxifen citrate Phosphorylation 98.3 Telenzepine dihydrochloride Cholinergic 68.9 Vincristine sulfate Cytoskeleton and ECM 89.2 Terfenadine Histamine 98.3 U-74389G maleate Cell Stress 51.6 U0126 Phosphorylation 68.9 U-83836 dihydrochloride Cell Stress 51.3 Vinblastine sulfate salt Cytoskeleton and ECM 92.8 Trifluoperazine dihydrochloride Dopamine 54.0 R(+)-Terguride Dopamine 57.1 (±)-Verapamil hydrochloride Ca2+ Channel 85.1 XK469 Apoptosis 75.9 S-5-Iodowillardiine Glutamate 52.8 Wortmannin from Penicillium funiculosum Phosphorylation 57.9 Thioridazine hydrochloride Dopamine 51.6 Tyrphostin A9 Phosphorylation 69.5 Zardaverine Cyclic Nucleotides 84.0 Thapsigargin Intracellular Calcium 94.8 (R)-(+)-WIN 55,212-2 mesylate Cannabinoid 97.7

TABLE 5 Genes represented in heat map FIG. 6-B Probe NM pubmed Symbol Complete name A_23_P10559 NM_001080395 AATK apoptosis-associated tyrosine kinase A_23_P60180 NM_005157 ABL1 c-abl oncogene 1, non-receptor tyrosine kinase A_23_P208389 NM_021913 AXL AXL receptor tyrosine kinase A_23_P253602 NM_001721 BMX BMX non-receptor tyrosine kinase A_23_P152024 NM_004383 CSK c-src tyrosine kinase A_23_P400716 NM_001010846 SHE Src homology 2 domain containing E A_23_P93311 NM_013993 DDR1 discoidin domain receptor tyrosine kinase 1 A_32_P88965 BX537651 DDR2 discoidin domain receptor tyrosine kinase 2 A_23_P5601 NM_001381 DOK1 docking protein 1, 62 kDa (downstream of tyrosine kinase 1) A_23_P124570 NM_005246 FER fer (fps/fes related) tyrosine kinase A_23_P502142 NM_002037 FYN FYN oncogene related to SRC, FGR, YES A_23_P48561 NM_005864 EFS embryonal Fyn-associated substrate A_24_P42755 NM_002019 FLT1 fms-related tyrosine kinase 1 (vascular endothelial growth factor/vascular permeability factor receptor) A_23_P213394 NM_182925 FLT4 fms-related tyrosine kinase 4 A_23_P61230 NM_004712 HGS hepatocyte growth factor-regulated tyrosine kinase substrate A_24_P929724 NM_015525 IBTK inhibitor of Bruton agammaglobulinemia tyrosine kinase A_23_P58419 NM_002253 KDR kinase insert domain receptor (a type III receptor tyrosine kinase) A_23_P376088 NM_017806 LIME1 Lck interacting transmembrane adaptor 1 A_23_P147431 NM_002350 LYN v-yes-1 Yamaguchi sarcoma viral related oncogene homolog A_23_P215275 NM_014916 LMTK2 lemur tyrosine kinase 2 A_23_P164918 NM_001080434 LMTK3 lemur tyrosine kinase 3 A_23_P14853 NM_002344 LTK leukocyte receptor tyrosine kinase A_23_P32955 ENST00000421804 MERTK c-mer proto-oncogene tyrosine kinase A_23_P86390 NM_003873 NRP1 neuropilin 1 A_23_P216779 NM_001007097 NTRK2 neurotrophic tyrosine kinase, receptor, type 2 A_24_P810476 ENST00000394480 NTRK3 neurotrophic tyrosine kinase, receptor, type 3 A_23_P22096 NM_153831 PTK2 PTK2 protein tyrosine kinase 2 A_23_P168836 NM_173174 PTK2B PTK2B protein tyrosine kinase 2 beta A_24_P320545 NM_002821 PTK7 PTK7 protein tyrosine kinase 7 A_23_P12363 NM_005012 ROR1 receptor tyrosine kinase-like orphan receptor 1 A_24_P229377 NM_001005861 RYK RYK receptor-like tyrosine kinase A_23_P374695 NM_000459 TEK TEK tyrosine kinase, endothelial A_23_P126416 NM_005424 TIE1 tyrosine kinase with immunoglobulin-like and EGF-like domains 1 A_23_P61633 NM_005781 TNK2 tyrosine kinase, non-receptor, 2 A_23_P54517 NM_006293 TYRO3 TYRO3 protein tyrosine kinase A_23_P215790 NM_005228 EGFR epidermal growth factor receptor A_23_P208369 NM_025194 ITPKC inositol-trisphosphate 3-kinase C A_23_P39682 NM_001079 ZAP70 zeta-chain (TCR) associated protein kinase 70 kDa

The present invention is not, in any way, restricted to the embodiments described herein and a person of ordinary skills in the area can provide many possibilities to modifications thereof without departing from the general idea of the invention, as defined in the claims.

The preferred embodiments described above are obviously combinable. The following claims define further preferred embodiments of the present invention.

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1. A differentiated cell population of endothelial cells derived from human pluripotent stem cells wherein a portion of the said endothelial cells express ephrin B2.
 2. The cell population according to claim 1 wherein at least 20% of the cells express ephrin B2.
 3. The cell population according to claim 1 wherein 50% to 75% of the cells express ephrin B2.
 4. The cell population according to claim 1 wherein the said endothelial cells further express the following marker Ac-LDL.
 5. The cell population according to claim 1 wherein the said endothelial cells further express one of the following markers: vWF, CD31, CD34, vascular endothelial cadherin, Flk-1/KDR.
 6. The cell population according to claim 1 wherein the said endothelial cells express one the following arterial endothelial cell genes: jagged 1, jagged 2, ephrin B1, Hey-2.
 7. The cell population according to claim 1 wherein the endothelial cells express one of the following markers: receptor protein tyrosine phosphatase, T-cell acute lymphocyte leukemia, N-cadherin, angiopoietin 1, DNA-binding protein inhibitor ID-1.
 8. The cell population according to claim 1 wherein the said endothelial cells have a low expression of venous genes such as EphB4, lefty-A and lefty-B.
 9. A differentiated cell population of endothelial cells derived from human pluripotent stem cells wherein a portion of the said endothelial cells express: receptor protein tyrosine phosphatase, T-cell acute lymphocyte leukemia, N-cadherin, angiopoietin 1, DNA-binding protein inhibitor ID-1 or least one of the following markers: vWF, CD31, CD34, vascular endothelial cadherin (VE-CAD), Flk-1/KDR for use in screening embryonic vascular toxicity or therapeutic compounds, preferentially a portion of at least 25%.
 10. The cell population according to claim 1 for use in medicine.
 11. The cell population according to claim 1 for use in screening embryonic vascular toxicity or therapeutic compounds.
 12. (canceled)
 13. A composition comprising the cell population described in claim
 1. 14. A kit for use in screening vascular toxicity or therapeutic compounds comprising the cell population described in claim
 1. 15. A fluidic system for use in screening therapeutic drugs or embryonic vascular toxicity, comprising a channel with a millimeter or micrometer dimension and a differentiated cell population of endothelial cells as described in claim 1 wherein the cells are cultured under physiologic shear stress.
 16. The system according to claim 1 wherein the cells cultured under physiologic shear stress are cells seeded and exposed to the media flow.
 17. The system according to claim 15 wherein the said cells produce glycocalyx.
 18. The system according to claim 15 wherein the channel comprises poly(dimethylsiloxane). 19.-26. (canceled)
 27. A device for use in screening vascular toxicity or therapeutic compounds comprising: endothelial cells derived from human pluripotent stem cells according to claim 1; and a fluidic system. 28.-32. (canceled)
 33. A method of promoting the viability of arterial endothelial cells comprising differentiation from human pluripotent stem cells with the following steps: a) culturing embryoid bodies in suspension for 4 days in differentiation medium comprising VEGF₆s; b) seeding the said embryoid bodies in a cell culture and sequentially exposing the cells to VEGF plus thymosin β4 for 3 days and VEGF plus thymosin β for 4 days and SB431542 for 11 days; c) isolating CD31+ cells and culturing these cells in endothelial cell medium containing SB431542. 34.-41. (canceled) 